Transcript Redox (download)

Oxidation and Reduction
Reactions that involve electron
transfer
Batteries and chemistry
What’s in a battery
A battery does work with electricity
A circuit is required
An electric current flows through the circuit
A chemical reaction provides the electricity
Energy and electricity
All chemical reactions involve energy
change
Reactions where energy is given out can
be made to provide the energy in the form
of electricity
Volta made the first battery (Voltaic cell)
All batteries involve electron transfer
Electron transfer involves
oxidation/reduction
Oxidation-Reduction (Redox)
Oxidation is loss of electrons
Na → Na+ + e-
Reduction is gain of electrons
Cl + e- = ClFormation of NaCl from elements is redox
Single displacement is redox
Zn atoms → Zn2+ ions (oxidized)
Cu2+ ions → Cu atoms (reduced)
Agents of redox
Oxidizing agent: causes oxidation of
another substance
– Cu2+ ions oxidize the Zn atoms
Reducing agent: causes reduction of
another substance
– Zn atoms reduce the Cu2+ ions
Nuggets of redox processes
Where there is oxidation there is always
reduction
Oxidizing agent
Reducing agent
Is itself reduced
Is itself oxidized
Gains electrons
Loses electrons
Causes oxidation
Causes reduction
Identifying oxidation and reduction
With elements forming ionic compounds
identifying oxidation and reduction is
usually straightforward
– Follow path of electrons from reactant to
product
What about covalent molecules and
reactions involving only compounds?
System of oxidation numbers is used
Oxidation numbers keep track of electrons
A numbers game
Daniell cell
An electrolytic cell which uses the
reduction of Cu2+ by Zn to produce a
voltage
– In left beaker is Cu and CuSO4
– In right beaker is Zn and ZnSO4
– Adding a “salt bridge” completes the circuit
and the reaction occurs
Zn + Cu2+ = Zn2+ + Cu
Galvanic cell: long distance
chemistry
Each metal in touch with a solution of its own ions
External circuit carries electrons transferred during the
redox process
A “salt bridge” containing neutral ions completes the
internal circuit.
With no current flowing, a potential develops – the
potential for work
Unlike the reaction in the beaker, the energy released by
the reaction in the cell can perform useful work – like
lighting a bulb
Odes to a galvanic cell
Cathode
– Where reduction
occurs
– Where electrons are
consumed
– Where positive ions
migrate to
– Has positive sign
Anode
– Where oxidation
occurs
– Where electrons are
generated
– Where negative ions
migrate to
– Has negative sign
Cell notation
Anode on left, cathode on right
Electrons flow from left to right
Oxidation on left, reduction on right
Single vertical = electrode/electrolyte boundary
Double vertical = salt bridge
Anode:
Zn →Zn2+
+ 2e
Cathode:
Cu2+ + 2e
→Cu
Volts and amps
Volt is the measure of potential – the driving
force to move electrons. Voltage depends on
the type of chemical process and not on the size
of the battery
Other forms of potential:
– Pressure moves air or liquids
– Temperature moves heat
– Chemical potential moves reactions
Amp is the flow of current. The size of the
current flowing can be increased by making the
electrodes larger (more reaction per second)
Measuring tendency for reduction
Reduction potential measures the tendency for a
substance to reduce another substance
The tendency is measured relative to some
standard – taken to be hydrogen
2H+ + 2e = H2
Standard reduction potentials are all measured
under the same conditions
– Negative value means that process is unfavourable
– Positive value means that process is favourable
Making predictions
Will the reaction
Zn + Cl2 = ZnCI2 proceed?
Zn + Cl2 = Zn2+ + 2ClPositive cell voltage means
reaction happens
What are the reduction potentials?
– Zn2+ + 2e = Zn
– CI2 + 2e = 2CI-
-0.76 V
+1.36 V
But…in the reaction Zn is oxidized
(reverse sign)
– Zn = Zn2+ + 2e
+0.76 V
Overall voltage:
0.76 V + 1.36 V = 2.12 V
Verdict: reaction proceeds
A rusty nail: corrosion and
electrochemistry
The rusting of a nail is an electrochemical
process
– Anode: Fe is oxidized to Fe2+
– Cathode: O2 is reduced to H2O
Why do things rust quicker in salt water?
Lithium batteries
Lithium has a very large negative
reduction potential
Li = Li+ + e….E = 3.04 V
The basis for light-weight, high energy
density batteries
– Low atomic mass of lithium
– High reduction potential
– Ability to make rechargeable batteries
Lead-acid batteries – a unique
system
Lead battery technology is 100 years old
–
–
–
–
Provides high current
Rechargeable
Inexpensive
Rugged
Oxidation: Pb + H2SO4 = PbSO4 + 2H+ + 2e
Reduction: PbO2 + H2SO4 + 2H+ + 2e = PbSO4 + 2H2O
Discharge
Overall: Pb + PbO2 + 2H2SO4 = 2PbSO4 + 2H2O
Recharge
Dry cell batteries
Acid dry cell
– Zn anode
Zn → Zn2+ + 2e
– MnO2 cathode
2MnO2 + 2NH4+ + 2e → Mn2O3 + 2NH3 + H2O
Alkali cell
– Zn anode
Zn + 2OH- → ZnO + H2O + 2e
– MnO2 cathode
2MnO2 + H2O + 2e → Mn2O3 + 2OH-
Fuel cells and the hydrogen
economy
A battery with the “electrolyte” supplied
from without
– Cathode:
2H2 → 4H+ + 4e
– Anode:
O2 + 4H+ +4e → 2H2O
– Overall
2H2 + O2 → 2H2O
Electrolysis – driving against the
stream
In the spontaneous process (left): electrons flow from left
to right (battery discharge powers cell phone)
Nonspontaneous process (right): apply voltage to
electrodes: electrons flow from right to left, reversing the
chemical reaction, restores potential energy to the bonds
(plugging phone into charger restores the battery)
Predicting spontaneity and the
activity series
A more active metal
will reduce a less
active metal ion
A less active metal
ion will oxidize a
more active metal
Redox in life
Corrosion
Fe + O2 → Fe2O3
Combustion
CH4 + O2 → CO2 + H2O
Bleaching
Cl + e- = Cl-
Biological systems
Respiration
Cytochrome c (Fe3+) + e- = cytochrome c (Fe2+)
Followed by:
O2 + 4e- + 4H+ → 2H2O
Metabolism
Ethanol → acetaldehyde → acetic acid → CO2 + H2O
Vitamin C and oxidation
Vitamins are organic compounds important for
maintaining health
Vitamin C is also easily oxidized (it is a reducing
agent)
Body produces free radicals which oxidize –
aging, cancer, cardiovascular disease
Antioxidants (like vitamin C) defend against
radicals
Question: should we take antioxidant
supplements?